It is well known to use digitally encoded signals over a radio link to open garage doors or unlock vehicle doors, for example, from a remote transmitter. Commonly, systems employing such control methods have a remote unit which may be carried in ones pocket or on a key chain and have a button which is pressed to issue a command signal. It is very desirable to make such systems secure from unauthorized use. This is especially important when the remote transmitter is used not only to unlock a vehicle door but also to unlock the vehicle ignition switch. When the signals are transmitted by radio, it is possible for a person using electronic eavesdropping to record the signals for later retransmission to operate the vehicle. More elaborate signalling procedures are needed to preclude such practices.
Encryption practices have been used for secure communications in areas of national security or for computer security, for example. To achieve the highest security, rather elegant methods have been adopted. A number of recent developments are described in the paper by Diffie, "The First Ten Years of Public-Key Cryptography", Proc. IEEE, Vol. 76, No. 5, May 1988, pp 560-577, which is incorporated herein by reference. The RSA public key system described therein is a widely accepted security measure which might be used for vehicle security. A precursor of the public key system, also described by Diffie, is the exponential key exchange. This is used along with other cryptographic techniques in the system described below.
The proposed system has a unique remote unit which can be positively verified by the base unit on the vehicle and cannot be imitated. The system further has the following objectives:
1. It has a high level of security even if all communications can be monitored and all aspects of the design are known. PA1 2. The remote unit cannot be copied or imitated even with physical access to the unit. PA1 3. One remote unit may be used with an unlimited number of base units. PA1 4. Compromise of one base unit shall not compromise other uses of the same remote unit. PA1 5. No operator input is needed beyond sign-up initiation. PA1 6. All functions other than radio transmission can be implemented by a single IC for each unit which can operate at very low power and complete the normal functions in a fraction of a second.